The invention relates to the dewatering of fibrous suspensions on a twin wire papermaking machine of the inverted type, i.e. machines wherein water is removed predominately upwardly by suction boxes located above the top forming wire of a substantially flat run of a continuously-moving sandwich formed by the top and bottom wires and a layer or web of fibrous suspension between them. This type of machine is particularly applicable to the production of multi-ply webs in which case subsequent plies are formed on a layer or web of fibrous suspension carried by one or more previously formed plies.
Known machines of this type include a large diameter forming roll having an open surface. Some initial upward drainage into the cellular surface of this roll may occur just behind the contact point between the roll and the top wire. The lowest point of the forming roll is level with or slightly below the running level of the bottom wire. This results in a slight "wrap" of the bottom wire around the forming roll and creates a narrow arcuate suction pressure zone behind the perforated forming roll, which depends on the tension of the bottom wire. This pressure per unit width of the roll, is indicated by the ratio of wire tension to roll radius. This tension or pressure tends not only to squeeze water from the suspension but also, unfortunately, to restrict the depth of fibre suspension that can traverse the forming zone and hence force the incoming flow to reverse direction forming a "puddle" in the nip between the forming roll and the bottom wire. In practice, the size of this puddle can be reduced by increasing the consistency of the fibre suspension thereby reducing its depth.
A second drainage may occur at the point where the top wire "wraps" the so-called underwire or table roll. Although the curvature of this zone is greater than the forming zone, their respective pressures are comparable due to a lower tension in the top wire. If the forming roll be raised in an attempt to reduce the wrap, there is a tendency for undesirable quantities of water to pass by the first table roll and to form a reverse flow "puddle" at a high curvature zone behind the table roll due to large pressure imposed by the so-called "autoslice" soon to be described. This problem has usually made it necessary to use high consistency suspension, i.e. to ensure that the fibre suspension entering the system has a low water content.
The autoslice may be a suction box or suction water scoop with an extended spout and a blade extending toward the forming roll. Its function is to collect water adhering to the upper surface squeezed out of the top wire, the fibre suspension in the forming zone and the table roll zone. The blade should just skim the top wire, but in practice it is often depressed into the wire run to increase the wrap of the table roll. This has the counter-effect of increasing the pressure at the slice lip which is already high due to its being substantially a "line" contact. Thus, apart from problems due to wear, the sheet must be formed so as to withstand high pressure at the slice lip. Hence all effective forming must take place between the forming roll and the table roll.
Further water removal and hence further consolidation of the fibre sheet has been obtained by the inverted suction boxes over the top wire, downstream from the forming zone.
Raising the forming roll tends to increase back flow at high pressure around the autoslice. If the autoslice also is raised in an attempt to relieve this pressure, the top and bottom wires tend to separate and the fibre sheet to disrupt, and so the remedy may lead to problems more serious than those it is designed to cure. Furthermore, suction boxes tend to wear to a concave formation and this also tends towards wire separation and sheet disruption.